JP-A-2000-112302 and JP-A-2003-316128 describe an image forming apparatus that detects abnormality in a charger.
An image forming apparatus described in JP-A-2000-112302 includes a single photosensitive member that is charged at the time of image forming and transfers a toner image onto a sheet. In the image forming apparatus, a charger leak detection circuit is provided on an output side of a high-voltage power supply unit connected to a primary charger, which charges the photosensitive member uniformly at a predetermined potential. The charger leak detection circuit detects occurrence of leak. In addition, in the image forming apparatus, a static eliminator leak detection circuit is provided on an output side of a high-voltage power supply unit connected to a static eliminator, which eliminates a transfer charge on the rear surface of the sheet to thereby separate the sheet attached to the photosensitive member from the photosensitive member. The static eliminator leak detection circuit detects occurrence of leak.
An image forming apparatus described in JP-2003-316128 includes four high-voltage power supply units and four chargers provided to correspond to photosensitive members of four colors. Each high-voltage power supply unit includes a leak detection circuit that detects leak of charging bias.
However, since a leak detection circuit is provided for each charger to detect abnormality in the above-described image forming apparatus, the circuit configuration is complicated and expensive.
JP-A-2007-178598 filed by the assignee of this application describes an image forming apparatus including a single discharge detection unit connected to a plurality of charging units or static eliminating units in parallel. In this image forming apparatus, the single discharge detection unit detects abnormality in the plurality of charging units or static eliminating units. With this image forming apparatus, the number of discharge detection units can be reduced, and thus the circuit configuration can be simplified and costs can be reduced.
However, in the image forming apparatus, since the discharge detection unit is connected to the plurality of charging units or static eliminating units in parallel, even if the discharge detection unit detects abnormality, the discharge diction unit could not specify a charging unit or static eliminating unit in which abnormality occurs. Accordingly, in this image forming apparatus, high-voltage power supply units are controlled so as to apply high voltage to the charging units or static eliminating units at different timings, and presence/absence of abnormality is detected for all of the charging units or static eliminating units, thereby detecting a charging unit or static eliminating unit, in which abnormality occurs. For this reason, in this image forming apparatus, it takes much time to detect abnormality in the charging unit or static eliminating unit.
To obtain a satisfactory printing result, each time a high-voltage power supply unit applies a voltage to a charging unit or static eliminating unit, the image forming apparatus checks whether or not abnormality occurs in the charging unit or static eliminating unit. Similarly to the above-described image forming apparatus, if it takes much time to detect abnormality in the charging unit or static eliminating unit, a print time may become longer, and a user may feel inconvenience. For this reason, there is a need for an image forming apparatus that can detect abnormality in a charging unit or static eliminating unit in a short time.